After you free Corvus from the Oubliette, you must follow him to the Realm of the Mighty and save the world.
Realm of the Mighty - Entrance
After you have eaten the Fygg that the Yggdrasil gives you, ask Sterling to take you to the Realm of the Almighty. As you arrive, it becomes the Realm of the Mighty.
When you arrive, walk onto the blue rectangle to make stairs appear. Go up. At the top, go west and open the chest to get a mini-medal. Then go east and enter the door.
Realm of the Mighty - L1
Go north, then go east at the fork in the path, then south to find a chest that contains a mini-medal. Go back to the fork in the path and go west. Go through the door there. Go east and up the stairs, then walk west from the stairs to find a chest that contains Mirror Armor. Go back the way you came and then go south to go through an arch in the stone. Go south as far as you can go, then go through the door there.
Realm of the Mighty - B2
Go north to find a chest that contains a Seed of Magic, then go to the west part of this room and and go up the stairs there.
Realm of the Mighty - B1
Follow the path north and go up the stairs.
Realm of the Mighty - L1, part 2
Go north and then follow the path eastward to reach some stairs. Go down. Follow the path until you reach a blue rectangle on the ground. Step on it and walk across the bridge that appears. Go east and climb the red vine, and go south from there to find a chest that contains a Seed of Skill. Then climb back down the vine and go north. Walk up the ramp and go west. Step on the blue rectangle and go up the stairs that appear. At the top, go north up the stairs to reach the next floor.
Realm of the Mighty - L2
Go east, then go all the way south. There is a blue chest in the southeast corner of this area. There are also jars west of there. Go to the southwest corner and go outside from there. Go east up the blue stairs and enter the big door.
Realm of the Mighty - L3
Go northeast and follow the path around south to go outside. Open the chest to get Yggdrasil Dew. Then go back inside. Go to the door in the middle and go through. Talk to Goreham-Hogg.
This time, Goreham-Hogg has 2470 HP. He's very weak to Light, somewhat weak to Ice, and is strong against Fire and very strong against Dark.
After you defeat him, go to the southwest corner of the room. Open the chest to get a mini-medal, then go west and go up the stairs.
Realm of the Mighty - L4
Go to the northeast corner and go down the stairs there. From there, go south and unlock the door, then go northeast. Open the blue chest there. Go back upstairs to L4 and go south from the stairs to go outside. Go west and walk across the blue bridge to find a blue chest. Then go back to the room where you defeated Goreham-Hogg. Go up the stairs there.
Realm of the Mighty - L5
Go east and cross the blue bridge, then keep going south to find a chest that contains a Ruby of Protection. Go north until you are on the northeast island, then go west to reach the northwest island. Go to the southeast end of this island and cross the bridge there to go east. Keep going until you reach a red vine. Climb up, then keep following the path. At the fork in the path, go north and open the blue chest, then go south until you reach the next area.
Go east across the blue bridge, then go south and climb down the long vine. At the bottom, go west and open the chest to get a Seed of Life, then go west and enter the door. Go north and open the blue chest, then go west and unlock the door there. This gives you a shortcut from the entrance to the upper floors of the Realm of the Mighty. Feel free to leave and heal up and come back later.
When you are ready, go back and climb up that long red vine, then go north, then west across the blue bridge. Go up the ramp and enter the door.
Realm of the Mighty - L6
Go west, then south to go outside. Climb up the red vine and open the blue chest in the area above. Then go south and go back down the vine, and go into the door at the bottom. Go north, then west, then north up some stairs, then west. Enter the jail cell and open the blue chest there. Go east and open the red chest to get a Seed of Therapeusis. Then exit the jail cells and go east to the northeast corner of the room. Use the exit there to go outside. Go west and go up the blue stairs. Enter the door and talk to Hootingham-Gore to fight him.
He has 1854 HP. He often casts Bounce, so it may be best to use only physical attacks. If you do want to try to use magic, Hootingham-Gore is very weak to Light, somewhat weak to Blast and Earth, and is strong against Dark and Wind.
Realm of the Mighty - L8
After the battle, go east. Keep going east and go down the stairs at the end. Keep going east and open the chest at the end to get a Rusty Sword. Then go west until you reach a ladder. Climb up to reach the next floor.
Realm of the Mighty - L9
Climb up the short ladder and open the chest to get a Spellward Circlet. Go south to exit the room. Go east until you reach a door. Go through. Go up the stairs and climb up the ladder.
Realm of the Mighty - L10
Go west and then go up the stairs that you find.
Realm of the Mighty - L11
Go south and walk off of the edge of the red carpet to fall into the air vent. Go east to find a chest that contains an Elfin Elixir. Then go west and talk to Goresby-Purrvis.
He has 2306 HP. He is weak to Wind, Blast, and Dark, and is strong to Fire, Ice, and Light. He uses the same attacks as before.
Afterward, go north and enter the palace. Go east or west and up the stairs. Go to the middle and enter the door. If you are ready for the final battles of the game, heal up and talk to Corvus.
The main danger in this battle is Corvus's use of Psyche Up, which greatly increases the damage that he can do. To succeed in this battle, make sure you consider Corvus's tension level when deciding whether to attack or defend. He has 2790 HP. He is very weak to Light, somewhat weak to Fire and Blast, is somewhat strong against Ice and Wind, and very strong against Dark. He can do regular attacks, lightning, Kaswoosh, Disruptive Wave, and can fly up into the sky to attack from above. He attacks twice per turn.
Afterward, leave the palace. There will be a brief cutscene. Go north and check on the sphere.
Barbarus has 3098 HP and is weak to Light, and strong against Fire, Ice, and Dark. He attacks twice per turn and can attack individuals or the whole party.
Afterward, get everyone ready because this is the final battle. Your party might get paralyzed during this battle, so give everyone some moonwort bulbs, or better yet, Softwort, if possible. If your party is at least level 35, but preferably around level 40 or higher, you should be able to win this battle. If you are ready, check on the sphere.
Corvus - Final Form
In his final form, Corvus has 4800 HP. He is very weak to Light, somewhat weak to Fire and Blast, somewhat strong against Earth, and very strong against Ice, Wind, and Dark.
Corvus might use Disruptive Wave, making it a gamble to use Fource spells or things like Kabuff on your party. He becomes more likely to use it the more you boost your party's attributes, so limit those boosts and you might be lucky enough to attack before your boosts are cancelled out.
Corvus has several breath attacks, such as Burning Breath, which can cause Paralysis (which is why you should bring Moonwort Bulbs or Softwort). He may use Eye Gleam, which inflicts sleep. He can do a combo attack of five hits that will each target a random person in your party. He can Pray to recover his MP or Meditate to recover 500 HP. His worst attack is Magic Burst that does a huge amount of damage to all. To make things even more difficult, Corvus might start using Psyche Up, raising his tension to increase the damage that he inflicts.
Use Multiheal, or better yet, Omniheal if available, to help keep everybody from dying. It will also help a lot if you have a character who can use Zing. If anyone in your party can use Whipping Boy, use it to protect your healer if necessary. Corvus might sometimes do critical hits, so you need to be prepared for the possibility that some of your characters might die. Try using Sap on Corvus to do more damage against him, but keep in mind that Sap doesn't always work on him. Carefully keep everyone healed as much as possible and do as much damage as you can per turn, and Corvus will eventually be defeated. Congratulations!! Enjoy the ending!